Selective corneal aberrometry

ABSTRACT

A method and system are disclosed for measuring and mapping the anterior surface topography of a cornea to determine corneal aberrations and an optimal ablation pattern for refractive and therapeutic surgery of the cornea. The invention stimulates a prospective ablation process with real-time visual feedback accurately portraying results of topographic surface alteration that would occur during an actual ablation procedure, thereby achieving minimal corneal aberrations and optimal image quality.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is filed under the provisions of 35 USC §371 and claimsthe priority of International Patent Application No. PCT/EP01/04142filed Apr. 11, 2001, which in turn claims priority of U.S. ProvisionalPatent Application No. 60/196,532 filed Apr. 11, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to surface profiling, and morespecifically, to a method and system for measuring and mapping theanterior surface topography of a cornea to determine corneal aberrationsand an optimal ablation pattern for refractive and therapeutic surgeryof the cornea.

2. Description of the Related Art

Various methods have been developed to reshape the cornea of the humaneye in order to correct for vision defects. Among these vision defectsare nearsightedness (myopia), with the unaccommodated nominal focusingplane falling before the retina; farsightedness (hyperopia), withfocusing plane beyond the retina; and the combination of defects knownas astigmatism, in which the cornea has a toroidal shape and there is noplane of best focus. The most common methods to correct for thesedefects are spectacles and contact lenses (hard, soft and gas permeabletypes) that provide the correct amount of refractive power to shift theunaccommodated focusing plane to its optimum position on the retina.However, glasses are worn externally and are not infrequently perceivedto be uncomfortable, inconvenient, or detracting from personalappearance.

Contact lenses are sometimes utilized when the use of glasses has beenconsidered to be undesirable, mostly for cosmetic reasons. However,contact lenses entail problems of their own in terms of possible eyeinfection and the necessity for time consuming procedures required tomaintain sterility and minimize contamination. More importantly, manypeople cannot tolerate the insertion of foreign objects on or in theireyes.

In response to a need for safe permanent correction of vision, withoutrecourse to glasses or contact lenses, several major surgical methodsfor vision correction have evolved. For instance, radial keratotomy(RK), involves surgical incision of the cornea, with deep radial cutsoutside the vision zone that cause a roughly predictable flattening ofthe cornea and a reduction in refractive power thereof, suitable forcorrecting low levels of myopia. Another procedure is corneal ablationwith an excimer laser (photo refractive keratotomy (PRK)) which isachieved by selectively ablating corneal tissue from the anteriorsurface of the cornea or by varying the front surface curvature of thecornea.

Before corrective surgery, a patient usually has several diagnostictests to determine the shape of the corneal surface. To date, measuringthe corneal surface involves the use of aberrometric devices thatgenerate an individual diagnostic set of data relating to the eyestructure to determine an ablation pattern for subsequent adaptation toan appropriate laser delivery system. However, systems and methods usedheretofore determine total aberrations of the optical system andintroduce not only corneal aberrations but also aberrations relating tovariable elements within the eye, such as the crystalline lens,accommodation of the lens, and vitreous structures. Thus, the measureddioptric values do not provide the option of differentiating aberrationscaused by the variable elements from that of corneal aberrationsoriginating on the anterior corneal surface. As such, a correctiveablation procedure for reshaping the cornea may not adequatelycompensate or may over-compensate due to aberration data from theundifferentiated variables.

Further, determining the topographic surface of the cornea with today'saberrometers does not provide sufficient data to measure the entire areaof the cornea because generally the aberrometers utilize only around 100measurement points with an accuracy of ±100 μm in the x and y plane.Considering that the x/y reference plane is centered about the pupilarmargin, and a corneal surface is usually about 8.5 mm in diameter, thereis not sufficient data to generate a topographic surface map with thenecessary precession.

Accordingly, at the present time, a need exists in the art for a methodand system that determine the topographic surface of the cornea withsufficient accuracy.

SUMMARY OF THE INVENTION

The present invention relates to a method and system for determining thesurface topography of a cornea to accurately represent the topography ofthe corneal surface and provide guidance for permanent corneal reshapingthat corrects for only corneal aberrations.

Additionally, the present invention relates to a system and method thatprovide a virtual image of the topographic surface of the cornea thatmay be combined with virtual components of the optical system todetermine the extent of corneal ablation required to achieve a positiveeffect on the optical properties of the eye.

Further, the present invention relates to a system and method thatprovide simulation of a prospective ablation process with real-timevisual feedback accurately portraying results of topographic surfacealteration that would occur during an actual ablation procedure toachieve minimal corneal aberrations and optimal image quality.

Still further, the present invention relates to a system and method thatprovide an accurate determination of corneal aberrations that do notinclude aberrations caused by internal structures within the opticalsystem, i.e., crystalline lens, aqueous and vitreous humor.

In one aspect, the present invention relates to a system for analyzingoptical properties of a structure, the system comprising:

-   -   means to topographically measure a surface of the structure and        produce a measured topography of the surface;    -   means to create a virtual surface corresponding to the measured        topography of the surface;    -   means to calculate paths of generated light beams contacting the        virtual surface; and    -   means to analyze the calculated paths.

In another aspect, the present invention relates to a system foranalyzing optical properties of a structure into which light waves enterthrough a surface of the structure, the system comprising:

-   -   means to topographically measure a surface of the structure and        produce a measured topography of the surface;    -   means to create and display a virtual structure substantially        corresponding to the structure;    -   means to create and display a virtual surface corresponding to        the measured topography of the surface of the structure, wherein        the virtual surface is combined with the virtual structure;    -   means to generate and display virtual light waves for passage        through the virtual surface into the virtual structure;    -   means to calculate paths of the virtual light waves passing        through the virtual surface; and    -   means to analyze the calculated paths to determine refractive        power of the virtual surface.

In still another aspect, the invention relates to a system for analyzingoptical properties of a structure into which light waves enter through asurface of the structure, for resolution of the light waves to create apicture, the system comprising:

-   -   means to topographically measure a surface of the structure and        produce a measured topography of the surface;    -   means to create and display a virtual structure substantially        corresponding to the structure;    -   means to create and display a virtual surface corresponding to        the measured topography of the surface of the structure, wherein        the virtual surface is combined with the virtual structure;    -   means for altering the virtual surface;    -   means to generate and display virtual light waves for passage        through the virtual surface into the virtual structure;    -   means to calculate paths of the virtual light waves passing        through the virtual surface; and    -   means to analyze the calculated paths to determine when the        virtual surface has been sufficiently altered to provide a        refractive power that shifts the virtual light waves to a        position within the virtual structure for resolution of the        virtual light waves.

In a further aspect, the invention relates to a method for analyzingoptical properties of a structure into which light waves enters througha surface of the structure, the method comprising:

-   -   measuring topographically a surface of the structure to produce        a measured topography of the surface;    -   creating and displaying a virtual structure substantially        corresponding to the structure;    -   creating and displaying a virtual surface corresponding to the        measured topography of the surface of the structure and        combining the virtual surface with the virtual structure;    -   generating and displaying virtual light waves for passage        through the virtual surface into the virtual structure;    -   calculating paths of the virtual light waves passing through the        virtual surface; and    -   analyzing the calculated paths to determine refractive power of        the virtual surface.

A further aspect of the invention relates to a method for analyzingoptical properties of a structure into which light waves enter through asurface of the structure, for resolution of the light waves to create apicture, the method comprising:

-   -   measuring topographically a surface of the structure to produce        a measured topography of the surface;    -   creating and displaying a virtual structure substantially        corresponding to the structure;    -   creating and displaying a virtual surface corresponding to the        measured topography of the surface of the structure, wherein the        virtual surface is combined with the virtual structure;    -   altering the topography of the virtual surface;    -   generating and displaying virtual light waves for passage        through the altered virtual surface into the virtual structure;    -   calculating paths of the virtual light waves passing through the        altered virtual surface; and    -   analyzing the calculated paths to determine when the altered        virtual surface has been altered sufficiently to provide a        refractive power that shifts the virtual light waves to a        position within the virtual structure for optimal resolution of        the virtual light waves to form a picture.

In a still further aspect, the present invention relates to a method foranalyzing optical properties of a structure, the method comprising:

-   -   measuring topographically a surface of the structure and        producing a measured topography of the surface;    -   creating a virtual surface corresponding to the measured        topography of the surface;    -   calculating paths of generated light beams contacting the        virtual surface; and    -   analyzing the calculated paths.

Therefore, the invention provides a method and a system that determinethe topographic surface of a cornea without necessarily combiningaberration data received from other structures within the optical systemand that provide a virtual simulation model utilizing the determinedtopographic surface for simulating results of a prospective ablationprocedure.

It is obvious that a visual display of the calculated surfaces, lightbeams or the like might be omitted as long as the necessary data areprovided to a person using the method or the system according to thisinvention.

Obviously, this method and this system are not limited to measurementsand ablation processes at an eye but may be used with any optical systemfocusing pictures in a body, which pictures are created by beamsentering said body through a surface. Especially, this method and thissystem may be used if the body comprises an internal structure beingoptically active.

Other aspects, features and embodiments in the invention will be morefully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the reflection pattern of a projection pattern comprised ofseveral concentric rings projected on a spherical surface.

FIG. 2 shows an example of a comparable reflection pattern in the eventof a cornea deviating from the spherical configuration.

FIG. 3 shows a schematic illustration of an arrangement for carrying outthe process according to the invention.

FIG. 4 is a block diagram of the system components of a preferredembodiment.

FIG. 5 shows a quality surface map according to the present invention.

FIG. 6 shows a corneal aberration map according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS THEREOF

Determining pre-operative corneal topographic abnormalities is a majordeterminant for ensuring predictability and stability of a subsequentrefractive surgical procedure. The present invention is based on thediscovery that determining and utilizing the corneal surface topographyis sufficient to analyze the optical system's deficiencies and provideinformation on the predictability and stability of these surgicalprocedures. Moreover, the present method and system enable thesimulation of an ablation procedure to determine an effective customablation pattern to produce a positive effect on the optical propertiesof the eye resulting in sufficient refractive power to shift anunaccommodated focusing plane to its optimum position on the retina forincreased visual acuity.

As stated above, the human eye is an optical system characterized byaberrations of several types. A contributing factor to these aberrationsis the fact that the human eye is not a rotationally symmetric opticalsystem. Additionally, corneas are never perfectly spherical, and assuch, it is important that the deviations therefrom be accurately andtopographically pre-determined for effective refractive correction.

Corneal topography is a method of measuring and quantifying the shapeand curvature of the corneal surface. Most topographic devices include aplacido disc made up of multiple circles, which is back lit or projectedonto the corneal surface. The resultant circular images are reflectedand captured with a video camera and then digitized for subsequentdisplay on a monitor. Using the mathematics of convex mirrors andmathematical algorithms, the image size is measured and quantified.Generally, any conventional corneal topography device may be utilized inthe present invention to generate illuminated concentric rings that areprojected onto the anterior corneal surface of a patient's eye. Theemitted light rays are reflected off the patient's cornea and at least aportion of the reflected light rays are captured by a lens and focusedonto an imagining system, such as the video camera.

In accordance with the present invention, a preferred topographicinstrument is the Tubingen Colour Ellipsoid Topometer (C-Scan)commercially available from Technomed Technology, Germany, as describedin U.S. Pat. No. 5,640,962, the contents of which are incorporated byreference herein, for all purposes. Specifically, the topographicinstrument provides x, y and z data to form a 3-D elevation map byutilizing at least three distinguishable recognition marks within aprojection pattern projected onto the cornea surface. Thedistinguishable markings are utilized for measuring the surfacetopography of the cornea and accomplished by a projection body which isilluminated, for example, by a white light source and has transparent,preferably ring shaped zones of different colors. In the alternative, itpossible to provide an arrangement of differently colored light sources,for example in form of an array of diodes.

Preferably, a graphics processing unit is utilized to capture thereflected data to form an image representing the reflective surfacewhich may be compared with a stored reference, or other knowninformation, to identify any distortions in the captured image. Thecomputer means measures the deflection of the light rings by thereflecting anterior corneal surface. Specifically, the distortion oflight reflected through the ring pattern by the conditions that exist onthe corneal surface is analyzed to determine the corneal topography andany deformation in the patient's cornea. The resulting data may bedisplayed as a corneal curvature map wherein different colors correspondto corneal power and curvature.

FIG. 1 shows schematically the image of an eye 1 of configuration 2 asregistered by an image detection system of the above-described type.Depicted within the interior of the circumferential line 2 is thecontour of the cornea 3. A placido disc is used to reflect rings ofdifferent colors onto the surface of the eye. Preferably, severalreflection rings 4 to 10 are generated by the cone device shown in FIG.3, wherein the rings are concentric to the Z-axis of the optical axis(camera-corneal apex). Located within the interior of these rings aretwo centering objects 11 and 12 that are described in more detailfurther below. The distance as well as the concentric and circulararrangement of the rings 4 to 10 correspond to the reflected image of ahealthy cornea with spherical surface.

In the case of astigmatism, the corresponding image of a deformed corneais shown in FIG. 2. The structures 4′ to 8′ represent also a mirrorreflection pattern of absolute concentric and circular projection rings.Their image is deformed by a non-spherical corneal surface. In part, theimages 5′ and 6′ of the respective and originally closed projectionrings have gaps 13, 14, 13′ and 14′ while other structures 7′ and 8′show significant dents 15 and 16. In a normal black/white photograph,which cannot be illustrated in the drawing any differently, the zone 15of the structure 7′ cannot unequivocally be associated to a respectiveprojection ring. This zone 15 could be associated to the projectionrings as formed by the reflection rings 5, 6 or 7 of a healthy cornea(FIG. 1). Advantageously, because the rings 4 to 10 and the structures4′ to 8′ are colored by a respective coloration of the projectionpattern the zone 15 can be unequivocally associated with the correctprojection ring.

In a preferred embodiment, a projection body in the form of a hollowcone or a hollow ellipsoid with transparent rings, in particular colorrings, is used in the side wall. FIG. 3 shows a schematic, sectionalillustration of an arrangement for carrying out the measuring processaccording to the present invention. Positioned in front of the eye 1,with the arched cornea 3, is a projection body 17. The projection body17 includes a cone-shaped hollow body, with a side wall 18 havingring-shaped, transparent and differently colored passages 19. Thecone-shaped projection body 17 is illuminated from outside by aring-shaped neon tube 20. The showing of two light beams emitting fromdifferently colored ring-shaped passages 21 and 22 and symbolized bylines 23 and 34 illustrates the projection of the ring structures uponthe cornea. The beams 23′ and 24′ reflected from the cornea 3 radiatethrough a pinhole diaphragm at the narrower end of the cone-shapedprojection body 17 and form an image in an image detector 26. The Z-axisis formed, as stated above, by the prolongation of the axis of the imagedetection unit relative to the surface being measured.

It is important in the described process to attain a good adjustment ofthe surface, for example the corneal surface, relative to the imagedetection unit in Z-direction. The image-forming qualities of the entirearrangement depend significantly on this distance. The adjustment indirection of the Z-axis is preferably carried out through scanning-in ofat least two centering objects in the projection pattern at a particularangle between their respective projection axes. The intersection of bothprojection axes yields the desired correct Z-position.

Centering objects 11 and 12 are scanned in at a certain angle betweentheir projection axes in a plane which coincides with the Z-axis.

Alignment of objects 11 and 12 provides for good adjustment of thecornea surface relative to the image detection unit in the Z-directionand allows the determination of the Z-position of the corneal surface 3and its evaluation. Laser beam 28 forms the centering object 11 e.g., onthe cornea and laser unit 27 is arranged outside the projection body 17.The projection body includes a small aperture in its wall for passage ofthe laser beam 28 which then intersects the Z-axis in the desired point.

Preferably, the image detector 26 includes a video cameracommunicatively connected to a graphics processing unit. The reflectionsignals from the corneal surface are converted into electrical signalsrelative to their intensity and the signal may be automatically executedin the graphics processing unit having a display monitor. Preferably adigital camera is utilized because the electronic signals resulting fromthe charge coupled devices (CCD) are not turned into an analog signal,but instead remain in the form which they are recorded and transferredto the memory of the graphics processing unit as digital signals. Theresults may be outputted in the form of isoreflection lines which referto those areas of the surface that have a same reflective power duringthe described measurement. Thus, the deviations from a standard ofmeasurement are visible directly and without requiring any furtherinterpretation of the measuring data.

To optimize the mapping of small corneal irregularities in cornealtopography for subsequent use in corneal and refractive surgery, thedata of small corneal irregularities are displayed in 3-D elevationmaps.

Amplification is needed to display those irregularities e.g., scars,central island after PRK, amount of tissue removed. This may be achievedby subtracting a best fit sphere or in the alternative, a softwareprogram, commercially available from C-Scan, Technomed Technology,Germany, may be utilized. The program specifically utilizes the line ofsight as a reference point (Z axis) combined with vector analysis andprovides a multiplicity of point evaluations ranging from about 100 to1000 to improve resolution. Further, this software tool is capable ofcompensating for errors which occur due to different image planes ofdifferent video topography pictures, such as before or after an ablationprocedure.

The resolution of the corneal map cannot, in general, exceed the numberof positions at which the data was acquired. It will be appreciated thatincreasing the resolution of the data can enhance the representation ofthe cornea scanning. To increase the resolution of the topographicsurface of the cornea, a method described in U.S. Pat. No. 5,900,924 maybe implemented, the contents of which are incorporated by referenceherein for all purposes.

The generated high resolution topographic 3-D map of the corneal surfaceis displayed on a graphics processing unit. To determine optimal customablation of the measured topographic surface, a virtual structure, suchas an eye structure is created on a geometric surface to form amanipulative three dimensional model of the eye and displayed on thegraphics processing unit. The virtual eye may be created by any methodknown in the art including, photographing components of an ideal eye andtexture mapping the photograph to develop a mathematical model of theeye. Further, an ideal virtual structure may be generated by a virtualreality drawing program such as, Optics Lab™, available from Science LabSoftware, Carlsbad Calif. Additionally, a virtual retina is created toprovide a surface for positioning a virtual image formed by convergenceof light waves.

After creation of a virtual ideal eye structure, a virtual image or theimage of the calculated topographic surface is connected or superimposedon the virtual ideal eye structure and displayed on the graphicsprocessing unit. Additionally, internal structures of the eye structuremay be included within the ideal virtual eye structure. For example, thepatient's iris may be mathematical generated by a ray tracing virtualreality program or photographed for digital recreation on the displayingmonitor. A digitized system is commercially available from IridianTechnologies, Moorestown, N.J.

Accordingly to the method and system of present invention, the opticalproperties of the topographical surface may be determined by thegeneration of virtual light waves that contact and/or pass through thevirtual topographic surface into the interior of the ideal eyestructure. A number of ray tracers software programs are currentlyavailable that convert a graphics processing unit or computer into anoptics workstation and generate different beam platforms for design andanalysis of optics system including, Optica™ which is commerciallyavailable form Wolfram Research, Inc., and Stellar™ from StellarSoftware, Berkeley, Calif.

The generated virtual light waves may be in several forms, such asparallel beams, beams having varying intensity or color, patterns andfrequencies.

Based upon data generated by the 3-D elevation map created by methodsdiscussed above, a corneal surface segment which is defined by an areabetween at least four (4)measured points is tested for visual acuity.Fast ray tracing, using the high resolution 3-D elevation data inconjunction with Snell's law may be used to describe the diffraction ofthe incident virtual light waves and the resulting images on the virtualretina. Generally, every measurable surface irregularity causes adeviation for an incident light beam (optical aberration) which can bedetected and quantified. A virtual light beam passing through themeasured corneal surface, the iris and projected into a virtual retinaresults in an offset in relation to an ideal beam, passing through anoptically ideal corneal surface. This offset can be processedquantitatively by projecting two points on the corneal surface whereby amathematical peak to valley analysis leads to an index for the cornealresolution. The index values are determine for a multiplicity of cornealsegments, preferably at least 100 segments, and more preferably between250 and 1000 segments, to determine an index for the best correctedcorneal visual acuity and deviations therefrom.

Accordingly, the index values are correlated and projected back onto thecorneal surface to generate a surface quality map. The surface qualitymap identifies the areas of the cornea with good or poor optical quality(lower or higher corneal aberration) and can display this information inboth percentages and/or a color coded map as shown in FIG. 5. Thissurface quality map can also be used to determine the functional opticalzones which show relative good optical quality and the map may be colorcoded according to visual acuity of more than 20/20.

Further, subtracting a best fitted sphere or asphere from the measuredtopographically surface will generate a difference map showing theamount and the location on the x and y plane of the cornea tissue thathas to be removed during surgery to achieve a surface topography thatimproves visual acuity. Thus, the corneal surface is corrected byremoving only tissue that will effect and improve visual acuity andavoiding ablation of corneal tissue that does not contribute to theimprovement of optical properties.

Further, data relating to contrast sensitivity may be generated byvarying the frequencies of the virtual light waves (sine or cosine wave)to calculate a projection that is comparable to known clinical contrastsensitivity charts. Thus, the high resolution 3-D topographic map may befurther utilized as a projection screen whereon individual cornealsegments are analyzed for a modular transfer function (MTF) and phaseshift function (PSF) to quantitatively generate a optical transferfunction. (Both the MTF and PSF may be created by the optics systemOptica™ described above)

Heretofore, clinically used contrast sensitivity represented acombination of the contrast (modular transfer function) and sensitivitywhich is a function of the retina. Both parameters are measured in oneclinical examination and subsequently included in a known vector visionchart. However, this is a subjective measurement and does not provideisolated information on the corneal surface unless a pre- andpost-operative measurement is taken. This may still not always beeffective because the patient must be extremely cooperative on bothexaminations to insure continuity in the alignment of the optical axis.

By the methods of the present invention the modular transfer functionMTF appears as contrast and the phase shift function PSF indicates theshift of the image relative to the optical axis (Z-axis). Basically, acontrast map can be generated by using the 3-D elevation data of thevideo topometer and a simulated projection of a virtual sinus wave.Practically, this can be accomplished by using a point spread functionand the projection of two points on the surface. Using the resultingpoint spread function g(k), the convolution F_(s)(x) is calculated usingthe equation:F _(s)(x)=½Σ_(k)(1+cos [α{x−k}])g(k)⁶with k=shift of spread function and x=local position on virtual retina.

The calculated projection of the sinus wave using real corneal elevationdata can be analyzed for the contrast behavior C using the equation:C=(I _(max) −I _(min))/(I _(max) +I _(min))²Where I_(max)=maximal light intensity (before passing cornea) andI_(min)=minimal light intensity (intensity on virtual retina).

A resulting graph or output, shown in FIG. 6, illustrates the contrast(modular transfer function) as a function of the sinus wave whereinvisual acuity is given by the intersection of the contrast curve withthe retinal sensitivity line.

The phase shift of the image is calculated to determine local tilt ofthe corneal surface for determining minimal amounts of decentration dueto ablation procedures. Decentration acts as a local tilt and thereforeleads to a shift of the image with regard to the optical axis. Differentfrequencies of sinus waves are transferred to the virtual retina, whichis defined as the focal plane resulting from approximately threemillimeters within the central area of the cornea. The phase shiftoccurs when the graph of a sin function A sin(ωt+φ) crosses the x axisand may be calculated by using a trigonometric identity for the sums ofangles wherein the sum of a sine and a cosine curve is equivalent to asine curve with a phase shift. A second graph shows the phase shift ofthe projected sinus wave relative to the optical axis as a function ofthe frequency. Both graphs, together with the included information,provides an optical transfer function (OTF). Further, the two graphs maybe combined with the surface quality map, which provide the opticalquality of each corneal point, to display the corneal opticalaberrations as shown in FIG. 6.

EXAMPLE I

A standard 4.5D PRK myopia correction with an optical zone diameter of6.5 mm, treated with an Apex plus (Summit, USA), was used to illustratethe obtained corneal aberration map (CAM), shown in FIG. 6, whichcombines the two graphs and surface quality map of FIG. 5. Theaberration map is divided into four sub charts including, a radius map,surface quality map, contrast graph and phase shift graph. At the lowerleft of the CAM, the distribution of radii is shown which indicates ahomogenous distribution. In the upper left of the CAM, the surfacequality map shows a central dark gray area (normally a colored amp)which indicates, quantitatively, the optical aberration prevailing in anormal 20/20 eye having a diameter of 4.2 mm after the 4.5D PRK. Theupper right graph shows “contrast” as a function of sinus wavefrequencies and indicates that contrast decreases rapidly for lowerfrequencies. The horizontal line represent, in this graph, thephysiological threshold and the intersection between the contrast curveand the physiological threshold correlates to the expected potentialvisual acuity for sinus wave structures. The phase shift graph in thelower right of the CAM shows the behavior of the phase shift as afunction of frequencies and indicates the phase shift is almostconstant.

The present map with the four charts can be used to provide informationnecessary to qualify the corneal aberration. While the corneal radiusmap is homogenous for the 6.5 mm diameter of the ablated optical zone,the surface quality map reduces the functional optical zone to 4.2 mmfor a given amount of myopic correction. Thus, for daylight conditions,a functional optical zone of 4.2 mm in connection with a small pupil,provide high visual acuity and good contrast behavior. The informationof the corneal aberration map can be used to guide a laser forcustomized ablation. Moreover, the corneal aberration map contains allthe information to achieve minimal corneal aberration, and thus, optimalimage quality.

Another embodiment of the present invention relates to the manipulationof the corneal surface to be performed by a virtual altering instrumentcreated within the graphics processing unit or computer. For a virtualablation procedure, a cut or removal of tissue caused by the virtualinstrument is monitored by tracking the tip of the instrument and thevirtual surface is reshaped to correspond to the virtual tissue removal.Subsequently, the visual acuity, as a result of each ablation cut, maybe determined by testing the altered virtual topographic surface of thevirtual cornea by methods described above. Accordingly, a pre-operativetest may be performed to determine the optimal ablation procedure and todetermine and achieve minimal corneal aberration.

In case of a medical application, in particular for determining thesurface topography of the corneal surface of an eye, a measuring devicewhich operates according the stated process, can be coupled directlyonto the operating microscope to enable the surgeon to recognize cornealareas having a deviation from the desired geometry and to immediatelyrender respective treatment.

1. A system for analyzing optical properties of a structure into whichlight waves enter through a surface of the structure, the systemcomprising: means for topographically measuring a surface of thestructure and producing a measured topography of the surface; means forcreating a virtual structure substantially corresponding to thestructure; means for creating a virtual surface corresponding to themeasured topography of the surface of the structure, wherein the virtualsurface is combined with the virtual structure; means for generatingvirtual light waves for passage through the virtual structure; means forcalculating paths of the virtual light waves passing through the virtualsurface; and means for analyzing the calculated paths to determinerefractive power of the virtual surface.
 2. The system according toclaim 1, wherein the structure is an eye having a topographic surface.3. The system according to claim 1 or 2, further comprising means formeasuring an internal structure within the structure and means forcreating a virtual internal structure substantially corresponding to theinternal structure.
 4. The system according to claim 3, wherein theinternal structure is an iris.
 5. The system according to claim 1,wherein said means for analyzing the calculated paths to determine therefractive power of the virtual surface, comprise means for creating avirtual focal area defining a virtual retina.
 6. The system according toclaim 5, wherein the calculated paths of the light waves merge at thefocal area.
 7. The system according to claim 1, wherein the light waveshave varying intensity.
 8. The system according to claim 7, wherein themeans for analyzing the calculated paths comprise means for analyzingthe phase shift of the beams of varying intensity, passing through thevirtual surface, wherein the phase shift is analyzed with respect to anoptical axis of the structure.
 9. A system for analyzing opticalproperties of a structure into which light waves enter through a surfaceof the structure for resolution of the light waves to create a picture,the system comprising: means for topographically measuring a surface ofthe structure and producing a measured topography of the surface; meansfor creating a virtual structure substantially corresponding to thestructure; means for creating a virtual surface corresponding to themeasured topography of the surface of the structure, wherein the virtualsurface is combined with the virtual structure; means for altering thevirtual surface; means for generating virtual light waves for passagethrough the virtual surface into the virtual structure; means forcalculating paths of the virtual light waves passing through the virtualsurface; and means for analyzing the calculated paths to determine whenthe virtual surface has been sufficiently altered to provide arefractive power that shifts the virtual light waves to a positionwithin the virtual structure for resolution of the virtual light waves.10. The system according to claim 9, wherein the structure is an eyehaving a topographic surface.
 11. The system according to claim 9,further comprising means for measuring an internal structure within thestructure and means for creating a virtual internal structuresubstantially corresponding to the internal structure and beingpositioned in the virtual structure behind the virtual surface.
 12. Thesystem according to claim 11, wherein the internal structure is an iris.13. The system according to claim 9, wherein said means for analyzingthe calculated paths to determine when the virtual surface has beensufficiently altered, comprise means for creating a virtual focal areadefining a virtual retina.
 14. The system according to claim 13, whereinthe calculated paths of the virtual light waves merge at the focal area.15. The system according to claim 9, wherein the light waves havevarying intensity.
 16. The system according to claim 15, wherein themeans for analyzing the calculated paths comprise means for analyzingthe phase shift of the beams of varying intensity, passing through thevirtual surface and the phase shift is analyzed with respect to anoptical axis of the structure.
 17. A system for analyzing opticalproperties of a structure, the system comprising: means fortopographically measuring a surface of the structure and producing ameasured topography of the surface; means for creating a virtual surfacecorresponding to the measured topography of the surface; means forcalculating paths of generated light beams contacting the virtualsurface; and means for analyzing the calculated paths.
 18. A method foranalyzing optical properties of a structure, the method comprising:measuring topographically a surface of the structure and producing ameasured topography of the surface; creating a virtual surfacecorresponding to the measured topography of the surface; calculatingpaths of generated light beams contacting the virtual surface; andanalyzing the calculated paths.
 19. A method for analyzing opticalproperties of a structure into which light waves enter through a surfaceof the structure, the method comprising: measuring topographically asurface of the structure to produce a measured topography of thesurface; creating a virtual structure substantially corresponding to thestructure; creating a virtual surface corresponding to the measuredtopography of the surface of the structure and combining the virtualsurface with the virtual structure; generating virtual light waves forpassage through the virtual surface into the virtual structure;calculating paths of the virtual light waves passing through the virtualsurface; and analyzing the calculated paths to determine refractivepower of the virtual surface.
 20. The method according to claim 19,wherein the structure is an eye having a topographic surface.
 21. Themethod according to claim 19, further comprising measuring an internalstructure within the structure and creating a virtual internal structuresubstantially corresponding to the internal structure and positioned inthe virtual structure behind the virtual surface.
 22. The method systemaccording to claim 21, wherein the internal structure is an iris. 23.The method according to claim 19, creating a virtual focal area, i.e. avirtual retina.
 24. The method according to claim 23, wherein thecalculated paths of the virtual light waves merge at the focal area. 25.The method according to claim 19, wherein the light waves have varyingintensity.
 26. The method according to claim 25, wherein the means foranalyzing the calculated paths comprise means for analyzing the phaseshift of the beams of varying intensity, passing through the virtualsurface wherein the phase shift is analyzed with respect to an opticalaxis of the structure.
 27. A method for analyzing optical properties ofa structure into which light waves enter through a surface of thestructure for resolution of the light waves to create a picture, themethod comprising: measuring topographically a surface of the structureto produce a measured topography of the surface; creating a virtualstructure substantially corresponding to the structure; creating avirtual surface corresponding to the measured topography of the surfaceof the structure, wherein the virtual surface is combined with thevirtual structure; altering the topography of the virtual surface;generating virtual light waves for passage through the altered virtualsurface into the virtual structure; calculating paths of the virtuallight waves passing through the altered virtual surface; and analyzingthe calculated paths to determine when the altered virtual surface hasbeen altered sufficiently to provide a refractive power that shifts thevirtual light waves to a position within the virtual structure forresolution of the virtual light waves.